4 Living Roofs in 3 Locations: Does Configuration Affect Runoff Quality or Quantity?

Four extensive living roofs in Auckland have been monitored over periods of 8 months to over 2 yrs for stormwater quantity and quality compared to conventional roofs at the same locations. Up to 56% cumulative retention was measured from living roofs with 50- 150 mm depth substrates designed to maximize water storage. Runoff rarely occurred from storms with less than 40 mm of precipitation. Peak flow was 62-90% less than the corresponding conventional roof per storm event. Flow path length through the drainage layer to vertical gutters and drainage layer material may influence peak flow control effectiveness and could be manipulated to increase mitigation. Neither living nor conventional roof surfaces produced elevated TSS or NOx. Zinc and copper mass loads were statistically comparable to runoff from the conventional roofs at the same location, although copper may be sourced from living roof substrates. Soluble Reactive Phosphorus and Total Kjeldhal Nitrogen are the predominant nutrients discharging at elevated concentrations from living roofs, with mass loads unlikely to be off-set by hydrologic control. Installing living roofs in nutrient sensitive receiving watersheds should consider a treatment train. Initial hypotheses regarding substrate organic matter characteristics to minimise contaminant leaching are suggested (e.g. carbon:nitrogen, Olsen phosphorus, cation exchange capacity and carbon content).